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From: David Xu <davidxu@FreeBSD.org>
Date: Mon, 22 Nov 2010 08:19:19 +0000 (UTC)
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Subject: svn commit: r215660 - user/davidxu/libthr/sys/kern
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Author: davidxu
Date: Mon Nov 22 08:19:18 2010
New Revision: 215660
URL: http://svn.freebsd.org/changeset/base/215660

Log:
  Merge from r215652:
      Use atomic instruction to set _has_writer, otherwise there is a race
      causes userland to not wake up a thread sleeping in kernel.

Modified:
  user/davidxu/libthr/sys/kern/kern_umtx.c

Modified: user/davidxu/libthr/sys/kern/kern_umtx.c
==============================================================================
--- user/davidxu/libthr/sys/kern/kern_umtx.c	Mon Nov 22 07:17:27 2010	(r215659)
+++ user/davidxu/libthr/sys/kern/kern_umtx.c	Mon Nov 22 08:19:18 2010	(r215660)
@@ -3565,9 +3565,8 @@ do_sem_wait(struct thread *td, struct _u
 	umtxq_insert(uq);
 	umtxq_unlock(&uq->uq_key);
 
-	/* Don't modify cacheline when unnecessary. */
 	if (fuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters)) == 0)
-		suword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters), 1);
+		casuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters), 0, 1);
 
 	count = fuword32(__DEVOLATILE(uint32_t *, &sem->_count));
 	if (count != 0) {